This invention relates to: thermoreversible recording materials which permits repeated reversible recording and erasure of visual information by the use of a heating means; to thermoreversible recording media using those thermoreversible recording materials; and to recording methods using those thermoreversible recording media.
A thermoreversible recording material has a property that its degree of transparency or transmittance at least with respect to visible light, varies in accordance with its thermal history. It is therefore possible, through the application of, for example, a thermal head or other heating means to such a thermoreversible recording material, to create a difference in the thermal history between a specific portion of the material and another portion, thereby creating a difference in the transmittance between the two portions for purposes of display or recording.
Thermoreversible recording materials according to the prior art are disclosed, for example, in Japanese Patent Kokai Publication No. S55-154198.
The thermoreversible recording materials disclosed in this publication are made of a matrix material of polymers, such as polyester, or resins, in which are dispersed organic compounds of low molecular weight, such as behenic acid.
FIG. 27 shows a hysteresis curve illustrating the variation of the light transmittance of a prior-art thermoreversible recording material against temperature, with transmittance on the vertical axis and temperature on the horizontal. Following is a description of the properties of thermoreversible recording materials according to the prior art with reference to FIG. 27. Points (A), (B), (C) and (D) in FIG. 27 show values of transmittance at respective temperatures. Specifically, (A) and (D) denote transmittances at room temperature (RT), (B) denotes transmittance at temperature T1, and (C) denotes transmittance at temperature T2.
In the vicinity of room temperature (RT), thermoreversible recording materials according to the prior art exhibit either low transmittance (A) (an opaque state) or high transmittance (D) (a transparent state) in FIG. 27, depending on the thermal history. If such a thermoreversible recording material is heated to a temperature T1 which is above a temperature T0, its transmittance will change from either (A) or (D) to (B), and if it is then cooled to room temperature, its transmittance, which in any case was (B), will stabilize at (D).
If, on the other hand, the thermoreversible recording material, the transmittance of which at room temperature was either (A) or (D), is heated above a temperature of T2 which is higher than temperatures T0 and T1, its transmittance, which was (A) or (D), will pass transmittance (B) and change to (C). That is, its transparency will decrease somewhat compared to (D). If it is then cooled to room temperature, its transmittance, which in any case was (C), will change to (A) and the thermoreversible recording material will stabilize at an opaque state (A).
The following examples of this property are disclosed in Japanese Patent Kokai Publication No. S55-154198 referred to above.
(1) When a thermoreversible recording material comprising a normal-chain copolyester of high molecular weight whose principal components are an aromatic dicarboxylic acid and an aliphatic diol, together with docosanoic acid, was heated to 72.degree. C. and cooled, it exhibited a stable transparency. It was possible to return it to an opaque state only by re-heating it to a temperature of 77.degree. C. or above.
(2) When a thermoreversible recording material comprising a copolymer of vinylidene chloride and acrylonitrile, together with docosanoic acid and a fluoride lubricant to improve fluidity, was heated to 63.degree. C. and cooled, it exhibited a stable transparency. It was possible to return it to an opaque state only by re-heating it to a temperature of 74.degree. C. or above.
(3) When a thermoreversible recording material comprising a copolymer of vinyl chloride and vinyl acetate, together with docosanol, was heated to 68.degree. C. and cooled, it exhibited a stable transparency. It was possible to return it to an opaque state only by re-heating it to a temperature of 70.degree. C. or above.
(4) When a thermoreversible recording material comprising a polyester together with docosanoic acid was heated to 72.degree. C. and cooled, it exhibited a stable transparency. It was possible to return it to an opaque state only by re-heating it to a temperature of 77.degree. C. or above.
Devices that record by image formation using heat-generating recording elements, on the other hand, are widely used because the heat-generating recording elements are inexpensive, the recording process is simple, and the price of the devices can be kept low. A well known method of this type is thermosensitive recording, which makes use of direct writing on thermally sensitive paper and finds application in telefax terminals and printers.
The recording paper used in this method is disclosed in Japanese Patent Kokoku Publication No. S45-14039, and uses a color-producing or developing reaction between a colorless die (leuco die), which serves as the electron donor, and a developer (a phenolic acid substance), which serves as the electron receptor, with a thermosensitive layer that assists the reactions between the two substances.
When heat energy from the heat-generating recording elements of a recording device is applied to this recording paper, the colorless die and developer melt and react to produce color. The difference in density between the portions in which color is produced and those in which it is not records characters and graphics in visible form.
However the range of temperatures within which thermoreversible recording materials according to the prior art will attain a transparent state is, as has been above described, extremely narrow: 5.degree. C. (77.degree.-72.degree. C.) for case (1), 11.degree. C. for case (2), 2.degree. C. for case (3) and 5.degree. C. for case (4), or at most only 11.degree. C.
To achieve a display of excellent contrast using a display device using a thermoreversible recording material, it is desired to provide a colored plate at the back of the display plate formed of the thermoreversible recording material, and, by selectively making the printed portion of the display plate transparent with a thermal head or other heating means, and leaving the background portions opaque, so that the color of the colored plate is seen only through the printed portion.
However, the range of temperatures within which a transparent state can be attained using the thermoreversible recording materials according to the prior art is, as has been above described, narrow, making it difficult to exert the requisite temperature control over the thermal heads or like heating means and to obtain stable transparency when images are to be repeatedly formed.
Again, the contrast (ratio of transmittances between the transparent and opaque states) for the prior-art thermoreversible recording materials is not particularly large, and further improvement has been desired.
There has not been any recording device which performs display in visible form on cards, sheets, or other recording media, that is to say printing, and recovers this printed portion by erasing it in its entirety, thereby enabling new printing on the recovered portion.
Thus thermosensitive paper, once printed, can only be destroyed should the information no longer be required, and in the present era of information and communication, an enormous amount of paper to be disposed of have resulted in a growing consumption of natural resources. There has also been a growing trend to encourage the use of regenerated paper as a way of conserving natural resources, but the manufacture of regenerated paper also consumes resources.
It is therefore desirable to develop a reusable recording medium and associated printing device capable of printing on a recording medium in directly visible form, recovering it by erasing the printed portion in its entirety, and printing again onto the recovered portion.